U.S. Pat. No. 5,812,679 and U.S. application Ser. No. 09/158,213 are incorporated herein by reference in their entirety.
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The present invention relates to an electronic hearing aid. More specifically, the present invention relates to an electronic damper for replacing the mechanical acoustic dampers used to smooth the frequency response of a hearing aid.
Generally stated, hearing aids include a microphone for transducing detected sound, an amplifier for amplifying the electronic signals received from the microphone, and an earphone for transducing the amplified electronic signals into sound for hearing by the hearing aid wearer. The microphones and/or earphones used in such hearing aids often do not have a flat frequency response, but, rather, have a generally flat frequency response with an undamped peak across a known frequency range.
Feedback is a potential problem in such hearing aids since the output of the hearing aid must of necessity be much greater than the input and since there is often leakage of sound from the interior of the ear to the exterior of the ear proximate the microphone input. The feedback problem is exacerbated by an undamped response peak of the microphone which represents a very-high-gain condition over a narrow frequency range. In many cases, the overall gain of the hearing aid is purposely reduced at most frequencies so that the gain at the frequency of an undamped peak will not produce feedback.
A reduction in quality of delivered sound typically accompanies an undamped peak. An undamped peak can also result in user discomfort where complex sounds have an energy concentration in the vicinity of the undamped peak. Such discomfort may be eliminated by reducing the overall gain of the amplifier. This approach, however, results in a loss of gain at the quiet sound level such that the hearing aid wearer does not receive the full benefit of the hearing aid amplification.
Acoustic damping has heretofore used mechanical dampers to smooth the frequency response of microphones and earphones (xe2x80x9creceiversxe2x80x9d) in order to smooth the overall frequency response of the hearing aid. The smooth response improves the overall performance of the hearing aid and helps prevent feedback.
In U.S. Pat. No. 3,930,560, Carlson and Mostardo described a fused-mesh mechanical damper. The damper described in that patent was subsequently made available as Knowles Electronic""s BF-series dampers in 330, 680, 1000, 15000, 2200, 3300, and 4700 (cgs acoustic) Ohm values. A 1979 application note titled xe2x80x9cSmoothing the ITE Frequency Response,xe2x80x9d and available from Knowles Electronics (Itasca, Ill.), described a xe2x80x9cmodel BF-1743xe2x80x9d damped coupling assembly incorporating that damper and designed to be mounted in the eartip of In-The-Ear (ITE) hearing aids. That damped coupling assembly provided a smooth response for the hearing aid earphone and permitted replacement of the damper when it became clogged with earwax or when a different value of damping resistance was desired. With that damped coupling assembly, a smooth hearing aid frequency response out to 16 kHz was practical.
Although mechanical damping mechanisms provide an improvement in the frequency response and performance of the hearing aids in which they are employed, such damping mechanisms are generally expensive and, further, are not entirely practical for some ears (especially in hot climates) since the damper elements tend to clog with earwax sometimes after only a few days. It is therefore desirable to have an alternative to such mechanical dampers.
A hearing aid is set forth that includes one or more hearing aid components that introduce one or more undesired undamped peaks into the frequency response of the hearing aid. One or more electronic damping filters are utilized to compensate for the undamped peak(s). Each such electronic damping filter has a notch filter response that includes an inverse peak across the frequency range of the undamped peak thereby electronically damping the frequency response so that the hearing aid output is rendered relatively free of the effects of the undesirable characteristics of the undamped peak(s).
In one embodiment of the invention, the hearing aid employs a microphone for transducing sound waves into electrical signals, an amplifier, and an earphone or xe2x80x9creceiverxe2x80x9d that transduces the amplified electrical signals from the amplifier into sound for the hearing aid wearer, the earphone and its coupling having a frequency response including a generally flat portion and at least one undamped peak. The undamped peak of the frequency response of the earphone occurs over a frequency range that is determined by the length of the sound outlet tube of the earphone. The microphone supplies electrical signals to an amplifier. The amplified signals are supplied to an electronic damper circuit that electronically damps the amplified output signal. The electronic damping circuit has a frequency response characterized by a generally flat portion and an inverse peak, the inverse peak occurring over a frequency range that generally corresponds to the frequency range of the undamped peak of the earphone. The resulting signal is an amplified signal that is generally unaffected by the undesirable characteristics of the undamped peak. This signal is supplied to a speaker that transduces the electrical signals into sound for the hearing aid wearer. The sound produced at the earphone corresponds to the sound received by the microphone but may have a frequency response that is modified to compensate for the type of hearing loss suffered by the intended wearer of the hearing aid. A further amplifier may be interposed between the electronic damping circuit and the earphone. Alternatively, the transduced signals from the microphone may be directly supplied to the damping circuit and the output of the damping circuit, in turn, amplified before being supplied to the earphone.
In another embodiment of the disclosed hearing aid, the electronic damping circuit is programmable to shift the frequency range and/or alter the magnitude of the inverse peak. This may be accomplished, for example, by using a low pass filter and a high pass filter. The filters may be adjusted so that their respective frequency responses overlap to provide a notch filter response, the position of the inverse peak in the frequency spectrum and the magnitude thereof being determined by the degree and location of the overlap in the low and high pass filter responses. The low pass and high pass filters may be formed as switched capacitor, Butterworth filters, the switching frequency of the filters determining the position and/or the magnitude of the inverse peak.
In a further embodiment of the hearing aid, the electronic damping circuit may be formed as an active bridged-T network circuit having a notch filter response. Programmability may be obtained by implementing the active bridged-T network circuit using virtual resistors comprised of switched capacitors wherein the frequency range and/or the magnitude of the notch response is determined by the frequency of at least one clock signal used to switch the capacitors of the filter.
In the overlapping Butterworth filter implementation and the bridged-T filters implementation of the damping circuit, programmability may be obtained by switching a plurality of capacitors in parallel to vary the capacitance values that determine the frequency characteristics of the filter.
In an even further embodiment, a digital signal processor is used to provide damping. The digital signal processor may execute low and high pass filters, and coefficients of such filters may be modified to alter the magnitude, shape and location of an inverse peak in the overall filter response. The coefficients may be modified via a user interface that communicates with the digital signal processor.
A method for producing multiple hearing aids is also set forth wherein the same electronic damping circuit topology can be used to dampen the frequency responses of two different hearing aids having different undamped frequency response characteristics. In accordance with the method, a first hearing aid is provided. The first hearing aid includes an earphone having at least one sound outlet tube to supply sound to the wearer. The earphone has a frequency response including a generally flat portion and at least one undamped peak wherein the undamped peak occurs over a frequency range that is dependent on the length of the outlet tube. A first programmable electronic damping circuit is provided for use in the first hearing aid. The programmable electronic damping circuit has a frequency response characterized by a generally flat portion and an inverse peak. The programmable electronic damping circuit is constructed using a predetermined circuit topology. The first programmable electronic damping circuit is then programmed so that the programmable frequency range of the inverse peak generally corresponds to the frequency range of the undamped peak to provide a hearing aid output signal that is generally unaffected by the undesirable characteristics of the undamped peak of the first hearing aid.
A second hearing aid is then provided. The second hearing aid includes an earphone having a frequency response including a generally flat portion and at least one undamped peak wherein the undamped peak occurs over a frequency range that is different from the frequency range of the undamped peak of the earphone of the first hearing aid.
A second programmable electronic damping circuit having the same circuit topology as the first programmable electronic damping circuit is then provided. The second electronic damper is programmed so that the programmable frequency range of the inverse peak generally corresponds to the frequency range of the undamped peak of the earphone of the second hearing aid. This results in a hearing aid output signal from the hearing aid that is generally unaffected by the undesirable characteristics of the inverse peak of the microphone of the second hearing aid.
Other objects and advantages of the present invention will become apparent upon reference to the accompanying detailed description when taken in conjunction with the following drawings.